Are you serious bungie?
You can join your friends fireteams without an invite on every console except for xbox one.
Why would you design the xbox one version this way? Having to send out invites to friends, or ask friends for invites is outdated. Even halo let you join your friends party without an invite.
Please fix this mess up, its not a very social game when its missing such a basic feature.
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Go to roster and on the left "fireteam", the little square above it is your fireteam privacy, make sure you're set to "public" or "friends only" and that who you're trying to connect to has the same thing. Hopefully it's a little bit of help, if you haven't messed with that already
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Is your NAT open?